The invention relates to a process for the preparation of polyamide granules at least comprising
a) polymerising at least one dicarboxylic acid and at least one diamine until a low-molecular prepolymer powder is obtained;
b) processing the prepolymer powder obtained in step (a) to form granules.
A similar process is disclosed in EP 254367 B1, in which a high-molecular polyamide-4,6 is prepared by liquid-phase polymerisation of 1,4-diaminobutane and adipic acid, flashing the resultant low-molecular polyamide (number-average molecular weight less than 10,000 g/mol) until a fine prepolymer powder is obtained and processing this powder to form granules. Subsequently, the granules are post-condensed until a high-molecular polyamide (number-average molecular weight greater than 10,000 g/mol) is obtained.
In the process according to EP 254367 B1, the prepolymer powder is processed to form granules (step b) by processes whereby the prepolymer powder is compacted by pressing the powder through small openings with the aid of pressure, whereupon the strands so formed are broken or are cut into granules. In particular, the powder is compacted with the aid of what is known as a rotating press. The press consists of a horizontal, flat, perforated mould on which at least two vertical collar wheels rotate. The powder is brought between the wheels, whereupon the powder is pressed into the perforations by the rotating wheels and so compacted to form compact polymer strands. Below the perforated disc, the formed polymer strands are broken by rotating knives into small pieces. In this way, a powder with a mean particle diameter (X50) of about 200-300 micrometers is granulated into granules about 1-5 mm long and about 3 mm in diameter. This technique, which is commonly applied in the industry, is characterised in that the strands are formed by the alternate application of pressure and relaxation to the ends of the strands that are nearest to the wheels, caused by rotation of the wheels.
A drawback of the process according to EP 254367 B1 is that the granules have low granule strength, which leads to the formation of fines, defined as particles whose average diameter is less than about 1.4 mm. A high fines content (typically 5-10% by weight on average for industrial processes) leads to a process with low efficiency and causes inter alia dust problems in the further processing and transport of the prepolymer granulate, necessitating for example suitable dedusting facilities. The formation of fine particles, originating from the granule, hereafter referred to as fragmentation, is a measure of the granule strength. It is desirable for a granule to have a high granule strength and so a low degree of fragmentation.
The object of the invention is a process for the preparation of polyamide granules that do not present the aforementioned drawbacks of the process according to the state of the art, in particular a process for the preparation of polyamide granules (granulate) with a higher granule strength than that of the state of the art, as well as the polyamide granules themselves.
Surprisingly, the inventors have found that the desired granules can be prepared by extruding the prepolymer powder obtained from step (a) at a temperature below the melting point of the prepolymer, measured at the chosen extrusion conditions.
Surprisingly, it has also been found that the process of the invention offers a number of additional advantages in comparison with the state of the art. For example, it was found that the prepolymer granulate obtained by the process of the invention contains far fewer fines than the granulate according to the state of the art. It was also established that the prepolymer granulate obtained by the process of the invention has a higher gas permeability and also that the granules had a lower porosity than the granules according to the state of the art.
A remarkable effect was established in the polymerisation of 1,4-diaminobutane and adipic acid to form a high-molecular polyamide containing at least 50% xe2x80x94NHxe2x80x94(CH2)4xe2x80x94NHxe2x80x94COxe2x80x94(CH2)4xe2x80x94COxe2x80x94 units. In EP 254367 B1, in order to obtain a high-molecular polyamide containing virtually equal numbers of carboxylic acid groups and amine groups, a large excess of 1,4-diaminobutane is utilised during the polymerisation step (a). This excess amounts to about 1-3% by weight. Use of a large excess amount of 1,4-diaminobutane has the drawback that relatively large amounts of the relatively volatile 1,4-diaminobutane compound volatilise out of the polyamide during post-condensation of the prepolymer granulate, which as a rule is effected at a temperature of 220-260xc2x0 C., so necessitating provisions to discharge the large amount of volatile compounds from the process. If too small an excess of 1,4-diaminobutane is employed, the polyamide obtained will contain an excess amount of carboxylic acid groups. An excess of carboxylic acids is highly undesirable in that, in the further processing of the post-condensed polyamide, it results in corrosion in the processing equipment, for example extruders and injection moulding machines, in low paintability and in unstable viscosity.
The inventors have found that, by post-condensing the granulate prepared according to the invention under identical conditions as in the state of the art, a high-molecular polyamide could be prepared in which there are present virtually as many carboxylic acid groups as amino groups, without using a large excess of 1,4-diaminobutane in step (a), in particular an excess smaller than about 1% by weight.
In the context of this application, low-molecular polyamide means a polyamide with a number-average molecular weight of maximum about 10,000 g/mol, in particular maximum about 5,000 g/mol. In the context of the present application, high-molecular polyamide means a polyamide with a number-average molecular weight of at least about 5,000 g/mol, in particular at least about 10,000 g/mol.
The process is suited for the preparation of all aliphatic and semi-aromatic polyamides in which during the preparation or processing a low-molecular prepolymer powder is formed, in particular during the preparation of the aliphatic polyamides polyamide-4,6, polyamide-4,8, polyamide-4,9, polyamide-4,10, polyamide-4,11 polyamide-4,12 polyamide-4,13, polyamide-4,14 and polyamide-6,6 and the semi-aromatic polyamides and polyamide 6,6/6,T, polyamide 4,6/4,T/4,I, polyamide-9,T, polyamide-12,T and copolyamides of the aforementioned aliphatic and semi-aromatic polyamides. More in particular, the process is suited for the preparation of high-molecular polytamide-4,6. The process is in principle suited for the processing to granulate of any polymer powder whose extrusion properties match those of a polyamide prepolymer powder, in particular a polyamide prepolymer powder as described in the examples.
As extruder use may be made of any type known to one skilled in the art, for example an single-screw type or a double-screw type, for example a counter-rotating type or a co-rotating type. The extrusion time preferably is 0.5-10 minutes. The extrusion pressure preferably is 1.106-1.5.107 Pa. Good results are obtained with a twin-screw extruder operated with an extrusion time of 1 minute and a pressure of about 5.106-1.107 Pa. Preferably, the pressure exerted on the extruder contents is not only relatively continuous but also relatively constant so that blockage of the extruder openings is minimised. It was also found that, preferably, in order to minimise blockage in the extruder, there should be no or hardly any dead spaces in the extruder, especially in the extruder head.
The prepolymer powder is preferably compacted in the presence of an amount of liquid. The liquid is preferably chosen from the group of water, methanol and ethanol. The liquid may also be a mixture of one or more liquids, for example a mixture of water and methanol. It is preferred for water to be chosen as the liquid.
The amount of liquid in the powder/liquid mixture is dependent on inter alia the polar character of the liquid, the average particle size and the particle size distribution of the powder, the type of extruder and the extruder settings. However, the amount of liquid can readily be determined by experiment by one skilled in the art. If the amount of liquid is too low, the powder/liquid mixture will have too high a viscosity for efficient processing by the extruder and the extruder may seize up. On the other hand, if the amount of liquid is too high, the viscosity of the mixture will be too low and no solid extrudate will be obtained. Good results are achieved with about 8-35% by weight of water, more preferably 12-18% by weight of water, relative to the weight of the total powder/liquid mixture. Water of crystallisation was taken into account in determining the amount of water.
The prepolymer may also be compacted in the presence of common additives known to one skilled in the art, in particular fillers, for example carbon fibres, glass fibres, glass beads, talcum, mica and wollastonite, flame retardants, impact modifiers, processing aids, for example Teflon, colouring agents, for example pigments, release agents, stabilisers and nucleating agents. This presents the added advantage that a separate compounding step, as often applied on an industrial scale after post-condensation of the prepolymer granules, can be omitted and so a more economical manufacturing process is obtained for high-molecular polyamide.
According to the invention, the prepolymer powder/liquid mixture is preferably extruded at a temperature, measured during extrusion and near the extrusion opening, which is below the boiling point of the liquid. If water is used as the liquid, the extrusion temperature preferably is lower than about 100xc2x0 C. Heating up of the prepolymer powder can be accomplished almost completely by friction in the extruder, depending inter alia on the extruder type and settings.
The granules and the granulate obtained with the process of the invention are particularly suitable for post-condensation.
The invention is illustrated by the following examples without being limited thereto.